Positron emission tomography (PET) is an imaging method in nuclear medicine based on the use of a weak radioactively marked pharmaceutical (a tracer) to image certain features of a body. PET images display the spatial distribution of the radiopharmaceutical enabling a doctor or clinician to draw conclusions about metabolic activities or blood flow, for example. Therefore, PET is a functional imaging technique that has applications in oncology, cardiology, and neurology, e.g., for monitoring tumors or visualizing coronary artery disease.
In PET imaging, a tracer agent is introduced into the patient to be imaged (e.g., via injection, inhalation, or ingestion). After administration, the physical and bio-molecular properties of the agent cause it to concentrate at specific locations in the patient's body. The actual spatial distribution of the agent, the intensity of the region of accumulation of the agent, and the kinetics of the process from administration to its eventual elimination are all factors that may have clinical significance.
During this process, a tracer attached to the agent will emit positrons, which is the anti-matter equivalent of the electron. When an emitted positron collides with an electron, the electron and positron are annihilated, resulting in the emission of a pair of gamma rays each having an energy of 511 keV and the two gamma rays traveling at substantially 180 degrees apart.
The spatio-temporal distribution of the tracer is reconstructed via tomographic reconstruction principles, e.g., by characterizing each detection event for its energy (i.e., amount of light generated), its location, and its timing. When two gamma rays are detected within a coincidence time window, they likely originate from the same positron annihilation event, and, therefore, are identified as being a coincidence pair. Drawing a line between their locations, i.e., the line-of-response (LOR), one can determine the likely location of the positron annihilation event. The timing information can also be used to determine a statistical distribution along the LOR for the annihilation based on a time-of-flight (TOF) information of the two gamma rays. By accumulating a large number of LORs, tomographic reconstruction can be performed to determine a volumetric image of the spatial distribution of radioactivity (e.g., tracer density) within the patient.
The detected coincidence events (called coincidences) can be classified into true coincidences and background events. The background events can be further subdivided into accidental coincidences and scattered coincidences. Accidental (or random) coincidences occur where the two gamma rays did not arise from the same annihilation event. Scattered coincidences occur when the two gamma rays did originate from the same annihilation, but where the true annihilation position does not lie on the LOR connecting the two photon positions. This can happen, e.g., when one gamma ray experiences Compton scatter within the patient, changing its direction of propagation.
Tomographic reconstruction has been widely applied to visualizing the anatomical information of patients. Tomographic reconstruction can be used in various modalities, including projection-based imaging, such as in X-ray computed tomography (CT), and emission-based imaging, such as in PET. Due to health concerns regarding exposure to radiation, doctors, scientist, and engineers in medical imaging strive to maintain radiation doses as low as reasonably achievable. This effort to maintain radiation doses as low as reasonably achievable motivates continued improvements in reconstructed image quality while decreasing the radiation doses and signal-to-noise ratios of the measured signals.
Accordingly, improved methods are desired for performing PET scatter correction and for improving the image quality of PET images by reducing noise and interfering signals. In PET imaging in particular, scatter correction (including outside-the-FOV scatter correction) plays a significant role in improving image quality while reducing the radiation exposure to patients.